Building a Takeover Bid Over Takeout

The weekend after the New York Stock Exchange delivered its bombshell about merger discussions with Germany's stock-market operator, two other exchange chiefs traded messages about the news.

"Are you thinking what I'm thinking?" said one of them, Jeffrey Sprecher of the IntercontinentalExchange Inc. On the other end of the line was Nasdaq OMX Group Inc.'s Robert Greifeld, a man who Mr. Sprecher had enjoyed meeting over the...